CS640: Introduction to Computer Networks

Aditya AkellaLecture 15TCP – II -

Connection Set-up and Congestion Control

2

TCP Packet

Source port Destination port

Sequence number

Acknowledgement

Advertised windowHdrLen Flags0

Checksum Urgent pointer

Options (variable)

Data

Flags from MSBto LSB: URG

ACKPSHRSTSYNFIN

Reliable, In-order,Connection oriented, Byte stream abstraction

3

Sequence and Acknowledge Numbers

• Sequence number byte num of first byte in payload

• Acknowledgement number– TCP is full duplex– Sequence number of next byte expected in

reverse direction

4

Advertised Window• Used for “flow control”

– Different from “congestion control”, which we will see in second half of today’s lecture

• Both sender and receiver advertise window– Sender action:

lastSent – lastACK <= Receiver’s advertised window

5

Establishing Connection:Three-Way handshake

• Each side notifies other of starting sequence number it will use for sending– Why not simply chose 0?

• Must avoid overlap with earlier incarnation

• Security issues

• Each side acknowledges other’s sequence number– SYN-ACK: Acknowledge

sequence number + 1

• Can combine second SYN with first ACK

SYN: SeqC

ACK: SeqC+1SYN: SeqS

ACK: SeqS+1

Client Server

6

Tearing Down Connection• Either side can initiate tear

down– Send FIN signal– “I’m not going to send any more

data”

• Other side can continue sending data– Half open connection– Must continue to acknowledge

• Acknowledging FIN– Acknowledge last sequence

number + 1

Client ServerFIN, SeqA

ACK, SeqA+1

ACK

Data

ACK, SeqB+1

FIN, SeqB

7

TCP State Diagram: Connection Setup

CLOSED

SYNSENT

SYNRCVD

ESTAB

LISTEN

active OPENcreate TCBSnd SYN

create TCB

passive OPEN

snd SYN ACKrcv SYN

Send FINCLOSE

rcv ACK of SYNSnd ACK

Rcv SYN, ACK

Client

Server

8

State Diagram: Connection Tear-down

CLOSING

CLOSEWAIT

FINWAIT-1

ESTAB

TIME WAIT

snd FINCLOSE

rcv ACK of FIN

LAST-ACK

CLOSED

FIN WAIT-2

snd ACKrcv FIN

delete TCBTimeout=2msl

send FINCLOSE

send ACKrcv FIN

snd ACKrcv FIN

rcv ACK of FIN

snd ACKrcv FIN+ACK

rcv ACK

Active Close

Passive Close

9

Congestion

• Different sources compete for resources inside network

• Why is it a problem?– Sources are unaware of current state of resource– Sources are unaware of each other

• Manifestations:– Lost packets (buffer overflow at routers)– Long delays (queuing in router buffers)– Can result in effective throughput less than bottleneck link

(1.5Mbps for the above topology) a.k.a. congestion collapse

10 Mbps

100 Mbps

1.5 Mbps

10

Causes & Costs of Congestion

• Four senders – multihop paths

• Timeout/retransmit

Q: What happens as rate increases?

11

Causes & Costs of Congestion

• When packet dropped, any upstream transmission capacity used for that packet was wasted!

12

Congestion “Collapse”• Definition: Unchecked Increase in network load

results in decrease of useful work done– Fewer and fewer useful packets carried in network

• Many possible causes– Spurious retransmissions of packets still in flight

• Classical congestion collapse

– Undelivered packets• Packets consume resources and are dropped elsewhere in

network

13

Congestion Control and Avoidance

• A mechanism which:– Uses network resources efficiently– Preserves fair network resource

allocation– Controls or Avoids congestion

14

Approaches Towards Congestion Control

• End-end congestion control:– No explicit feedback

from network– Congestion inferred

from end-system observed loss, delay

– Approach taken by TCP– Problem: approximate,

possibly inaccurate

• Network-assisted congestion control:– Routers provide

feedback to end systems• Single bit indicating

congestion (SNA, DECbit, TCP/IP ECN, ATM)

• Explicit rate sender should send at

– Problem: makes routers complicated

• Two broad approaches towards congestion control:

15

End-End Congestion Control• So far: TCP sender limited by available buffer size

at receiver– Receiver flow control– “receive window” or “advertised window”

• To accommodate network constraints, sender maintains a “congestion window”– Reflects dynamic state of the network– Max outstanding packets ≤ min {congestion

window, advertised window}

• When receiver window is very large, congestion window determines how fast sender can send– Speed = CWND/RTT (roughly)

16

TCP Congestion Control• Very simple mechanisms in network

– FIFO scheduling with shared buffer pool– Feedback through packet drops

• End-host TCP interprets drops as signs of congestion and slows down reduces size of congestion window

• But then, periodically probes – or increases congestion window– To check whether more bandwidth has become

available

17

Congestion Control Objectives

• Simple router behavior

• Distributed-ness

• Efficiency: xi(t) close to system capacity

• Fairness: equal (or propotional) allocation– Metric = (xi)2/n(xi

2)

• Convergence: control system must be stable

18

Linear Control• Many different possibilities for reaction to

congestion and probing– Examine simple linear controls

• Window(t + 1) = a + b Window(t)• Different ai/bi for increase and ad/bd for decrease

• Various reaction to signals possible– Increase/decrease additively– Increased/decrease multiplicatively– Which of the four combinations is optimal?

• Consider two end hosts vying for network bandwidth

19

Additive Increase/Decrease

T0

T1

Efficiency Line

Fairness Line

User 1’s Allocation x1

User 2’s Allocatio

n x2

• Both X1 and X2 increase/ decrease by the same amount over time– Additive

increase improves fairness and additive decrease reduces fairness

20

Multiplicative Increase/Decrease

• Both X1 and X2 increase by the same factor over time– Extension from

origin – constant fairness

T0

T1

Efficiency Line

Fairness Line

User 1’s Allocation x1

User 2’s Allocatio

n x2

21

Convergence to Efficiency

xH

Efficiency Line

Fairness Line

User 1’s Allocation x1

User 2’s Allocatio

n x2

22

Distributed Convergence to Efficiency

xH

Efficiency Line

Fairness Line

User 1’s Allocation x1

User 2’s Allocation x2

a=0b=1

a>0 & b<1

a<0 & b>1

a<0 & b<1

a>0 & b>1

23

Convergence to Fairness

xH

Efficiency Line

Fairness Line

User 1’s Allocation x1

User 2’s Allocatio

n x2

xH’

24

Convergence to Efficiency & Fairness

• Intersection of valid regions• For decrease: a=0 & b < 1

xH

Efficiency Line

Fairness Line

User 1’s Allocation x1

User 2’s Allocatio

n x2

xH’

25

What is the Right Choice?

• Constraints limit us to AIMD– Can have

multiplicative term in increase(MAIMD)

– AIMD moves towards optimal point

x0

x1

x2

Efficiency Line

Fairness Line

User 1’s Allocation x1

User 2’s Allocatio

n x2

26

Summary• Significance of fields in TCP packet

– TCP is full duplex– Data can go in either direction

• TCP connection set-up – three-way hand-shake– Also, teardown

• Costs of congestion– Delay, loss, useless work…– Cure: congestion control

• TCP uses AIMD congestion control